Recently, there has been desired a quantum computer using quantum superposition as a bit to memorize arithmetic information. The quantum computer operates a qubit memorizing information to enable a basic gate of the quantum computer. In particular, the basic gate of the quantum computer corresponding to single qubit is broadly classified as either a rotation gate to shift populations (i.e., square of probability amplitude) of a state |0> and a state |1> or a phase gate to change a relative phase between the state |0> and the state |1>. That is, the phase gate is one of single-qubit gates which are basic gates of the quantum computer and also important structural elements of the quantum computer.
Further, as a mounting method of the quantum computer, there has been widely known a method. The method uses a system which has some energy levels of a material having a long duration of a quantum superposition as a qubit and apply some coherent laser at a frequency which is in the vicinity of a resonance frequency of the energy level to the system thereby perform gate operation.
In the gate operation, efficiency decreases when an eigenstate of the system showing a qubit transitions to another eigenstate during the gate operation. An adiabatic condition to be expressed with [Expression 1] is used as an index to represent a probability of the transition.|{dot over (φ)}|φ′|<<|ωφ−ωφ′|
The adiabatic condition of a transition from an eigenstate |φ> to another eigenstate |φ′> is defined by the following expression using an eigenvalue ωθ of the eigenstate |φ>. Here, the dot shows time-derivative. Further, in the present specification, the following equation is defined as a non-adiabatic effect.
                          〈                              ϕ            .                    |                      ϕ            ′                          〉                                                  ω          ϕ                -                  ω                      ϕ            ′                                      ≡  A
It has been known that the gate operation is efficient without excitation of a lower level to an upper level in a resonance state in which a degree of detuning is sufficiently smaller than an energy level difference of a material, the degree of detuning being a difference between an energy value of a laser photon and the energy level difference of the material. In particular, it has been known that high efficiency is obtained when two-photon detuning is zero, i.e., a two-photon resonance improves the efficiency. The two-photon resonance appears when the differences between the respective energy values of two laser incident beams and the two-energy level differences, i.e., the degree of the two-photon detuning is zero.
The following [expression 2] to express a qubit state is changed by a phase gate of the quantum computer. In the phase gate, when the two-photon detuning of the two incident laser beams is zero, a phase shift does not arise and the gate operation cannot be performed in the two-photon resonance. Therefore, the gate operation efficiency is drastically lowered.
                              ϕ          =                      a            +                          ⅈ              ⁢                                                          ⁢              b                                      ⁢                                  ⁢                              phase            ⁢                                                  ⁢            of            ⁢                                                  ⁢            ϕ            ⁢                          :                        ⁢                                                  ⁢                          θ              ϕ                                =                      arctan            ⁡                          (                              b                a                            )                                                          [                  Expression          ⁢                                          ⁢          2                ]            